The present invention relates to a data transmission system used for transmission of a message among a plurality of terminal units through a loop transmission line.
In a conventional transmission system including a loop transmission line, in order to determine the time required for transmission and reception of a message at each of a plurality of transmission control units connected in a loop and carry out countermeasures for a partial fault of the loop, one of the transmission control units is assigned to a master transmission control unit while the others are assigned to slave transmission control units. The master transmission control unit monitors a transmission status of the entire loop to determine the transmission and reception timing and to execute the countermeasures for the fault of the loop. As a result, a large-size master transmission control unit is required, and when the master transmission control unit is out of order, the operation of the transmission system is completely stopped.